Schematic representation of the two-phase-exchange model of fluids confined in saturated porous matrices. The fluid adsorbed at surfaces (population pa and correlation time τa) is distinguished from the bulk-like phase (population 1−pa and correlation time τb). Molecular mobilities within and exchange kinetics between these phases determine the dynamics of fluid molecules. The mean desorption time in the adsorbed phase is denoted τdes. The limits T1(ω)≫τ≫τdes (fast exchange on both the relaxation and correlation time scales) and T1(ω)≫τdes≫τ (fast exchange on the relaxation time scale, slow exchange on the correlation time scale) are of particular interest.